J . Phys. Chem. 1984, 88, 5042-5048
5042
-
react with a variety of oxygen carriers, for example B
+ H20
or B
+ C3H60
BO(A211)
+ H2(1Z:g+)10
BO(A211)
+ products”
where C3H60 = 1,2-epoxypropane. Those chemiluminescent emissions were recorded with much lower spectral resolution in molecular beam experiments, so that the rotational temperatures could not be ascertained. While all of our experiments were performed under multiple-collision conditions (note the relaxed rotational temperature), vibrational relaxation was not achieved even in the presence of excess oxygen atoms and many free radicals. In view of the observed low rotational temperature (hence low translational temperature), it remains for highly exoergic reactions to be the direct source of the excited electronic and vibrational emitters. Only two reactions involving B/H species have sufficient energy to account for the observed u’ = 10 in the A state:12 0
+
BH
-
LO-B-HI
-
OBx
+
HD
and
(Were BO produced in the X state, the enthalpy releases at 400 K would be 113.26 kcal/mol for the first and 107.53 kcal/mol for the second reaction.) However, the latter is less plausible for generating high states of vibrational excitation in BO*, since it (10) (a) J. L. Gole and S.A. Pace, J . Phys. Chem., 85,2651 (1981);(b) A. W. Hanner and J. L. Gole, J . Chem. Phys., 73, 5025 (1980). (1 1) P. Davidovitz et al., Chem. Phys. Lett., 86,491 (1982),and previous
communications.
(12)For estimates of enthalpies of boron-containing species, refer to M. J. S.Dewar and M. L. McKee, J . Am. Chem. Soc., 99,5231 (1977),and the JANAF Tables.
is unlikely that the ejected H2 would leave without significant internal energy. Further, with respect to the three-atom reaction, one could argue that the electronically excited species are generated directly, rather than by resonant transfer, from very high vibrational levels in the 22+state. The intermediate, presumably linear structure 0-B-H13 must incorporate the electronic orbital angular momentum from the oxygen, which would readily correlate with the n-state of the product BO. For the production of BO(A211) by reaction of boron atoms with oxygen carriers, Hanner and Golelob proposed that resonant transfers from the X state (u” = 17) contribute significantly to enhance the population of the u’ = 4 level. Also, at the crossing of the X2B+ and A211potential curves ( u ” = 21; u’ = 9), another close coincidence appears, which leads to a second perturbation. Further investigations are needed to determine the origin of the stepwise features in the vibrational-state populations. No mechanisms have yet been established which account for the production of BO in the @system. One possible explanation is the occurrence of extensive energy pooling among the various excited species. Thus, if oxygen atoms attack BH(AIII), which is present in significant levels, then BO(B2Z+) could be produced.
Acknowledgment. This study was supported by the ARO under Grant No. DAAG29-81-K-0037. We thank Professor T. A. Cool for a loan of his standard black-body source and Professor R. F. Porter for the use of his optical pyrometer. We greatly appreciate the revised potential energy curves and detailed calculations of vibrational states sent to us by Dr. J. H. Michels (United Technologies Research Center). Registry No. H,B.N(CH,),,75-22-9;N(CH,),,75-50-3;0,, 778244-7;pyridine borane adduct, 110-51-0;tetrahydrofuran borane adduct, 14044-65-6;tert-butylamineborane adduct, 7337-45-3; dimethyl sulfide borane adduct, 13292-87-0. (13)IR spectrum of matrix-isolated HBO was reported by E. R. Lory and R. F.Porter, J . Am. Chem. SOC.,93, 6301 (1971). In the gas phase, the spectrum of the analogous halogenated species ClBO was analyzed by K. Kawaguchi, Y . Endo, and E. Hirota, J. Mol. Spectrosc., 93, 381 (1982).
Oscillating Convective Effects in SF6-Ar Laser-Heated Mixtures S. Ruschint and S.
H.Bauer*
Department of Chemistry, Cornell University, Zthaca, New York 14853 (Received: February 14, 1984)
Periodic temperature variations were observed in vertical absorption cells which were filled with mixtures of SF6 and Ar. The cells were irradiated from below with a continuous C02laser beam of moderate intensity (3-15 W). The onset of oscillations and their dependence on mixture composition and laser irradiation intensity were measured. A qualitative model is proposed to account for this periodic behavior. Allowance for temperature oscillations must be made in developing thermochemical or kinetic parameters from laser-powered homogeneous pyrolysis experiments.
Introduction The nonlinear behavior of absorption of infrared radiation by molecular gases has been the cause of a wide range of effects. When the absorbing gas is mixed with a suitable amount of inert gas so as to raise the overall pressure to several torr or higher, thermalization occurs in less than a millisecond and a unique local temperature can be defined for all molecular degrees of freedom. As a consequence, mixtures of gases can be heated by laser radiation to temperatures which are determined by the composition of the species present. One practical application of this type of heating is laser-powered homogeneous pyrolysis.’ Thus, by collisional energy transfer, chemical reactions which require high ?.Permanent address: Faculty of Engineering, Department of Electron Devices, Tel-Aviv University, Ramat-Aviv 69978,Tel-Aviv, Israel.
temperatures can be investigated for reactants which do not absorb at the available laser wavelengths, avoiding perturbations due to heterogeneous reactions which may take place on the hot walk2 Detailed information and analyses of the temporal and spatial temperature distributions in laser-heated absorption cells are therefore of considerable interest. When the absorption cross section is high most of the incident radiation is deposited close to the cell entrance window. Then large temperature gradients are induced which, in vertical configuration, lead to convective flows. Such flows cause mixing, for in addition to diffusion the mass of the hot gas rises to the (1) W. M. Shaub and S. H. Bauer, I n f . J . Chem. Kine?.,7,509 (1975). (2) M. R. Berman et al., J. Am. Chem. SOC.102,5692(1980);K. E. Lewis et al., J . Phys. Chem., 84,226 (1980).
0022-3654/84/2088-5042$01.50/00 1984 American Chemical Society
Convective Effects in Laser-Heated Mixtures
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 5043
@$
P.M. Z
L
O-RING
w;::
SEAL
THERMOCOUPLE
Figure 1. Schematic of overall experimental setup.
upper part of the cell where the laser radiation is weaker. Under extreme conditions, i.e., when the laser radiation is completely absorbed in the lower portions of the cell, the high temperatures are initially confined to the lower strata of the cell; then for a range of mixture parameters and irradiation intensities the system does not attain steady state. A regular oscillatory pattern of the temperature as a function of time develops, with periods of the order of 1-4 s. In this laboratory Richards first observed oscillations in the temperature in gas cells heated from below by CO, laser radiation when the beam was completely a b ~ o r b e d . ~ Oscillatory behavior of convective patterns in fluids has been reported by several authors. These observations were reviewed by BUSS^^^ in a comprehensive article. In most cases the systems studied were extended horizontally which allowed the natural formation of convective roll or “cell” patterns.4b Time-dependent stabilities were found in the transition between steady bimodal convection and turbulent convection. The instabilities assumed the form of wavelike periodic changes in the cell patterns. Oscillations usually appeared at high Rayleigh and Prandtl numbers5” (Ra= 5 X lo4;Pr = lo), their onset being principally controlled by the former. Oscillations were observed at lower Prandtl numbers in the transition between turbulent and steady threedimensional convection’ and in gaseous fluids.* Oscillatory convective effects have also been investigated theoretically with analyses which ranged from simple analytical modelsgto extensive numerical calculations.1° The experiments described below were performed with a cylindrical absorption cell oriented vertically. Its diameter to height ratio was approximately 1:3. This fact, together with the directly measured temperatures, suggests that all the temporal variations occured within one convection cell, delimited by the dimensions of the absorption cell.
Experimental Section Apparatus Configuration. Figure 1 illustrates schematically the experimental arrangement, which consisted of a grating tuned C 0 2laser, apertures, absorption cell, five thermocouples, and the data acquisition system. The laser was a continuous wave axially discharged unit operated at a relatively low intensity (
